<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>

    <input class="a" type="text" disabled>

    <!-- 单选或者复选框中  默认选中  checked -->
    <input class="b" type="checkbox" checked>

    <select class="c" name="" id="">
        <option value="1" >1</option>
        <option value="2" selected>2</option>
        <!-- 下拉菜单默认选中  selected -->
        <option class="d" value="3" >3</option>
    </select>

    <script>

        // disabled    禁用

        //  checked    单选或者复选框选中

        //  selected   下拉菜单默认选中

        var oInp1 = document.querySelector('.a') ;

        console.log(oInp1.disabled) ;  // true / false

        var oInp2 = document.querySelector('.b') ;

        console.log(oInp2.checked) ;   // true / false


        // 下拉菜单某个选项是否被选中的时候   获取对应的这个option
        var oOpt = document.querySelector('.d') ;
        console.log(oOpt.selected);


        var oSel = document.querySelector('.c') ;
        console.log(oSel.value);



        setTimeout(function(){
            oInp1.disabled = false;
            oInp2.checked = false;
        } , 4000)

    </script>
    
</body>
</html>